Despite the expanding arsenal of new targeted agents to treat castrate-resistant prostate cancer (CRPC), the disease remains incurable, with nearly half of men with this form of prostate cancer (PCa) developing bone metastases at two years. Metastatic bone disease carries a one year survival rate on the order of 40%. New targeted therapies vary widely in mechanism, from those that inhibit androgen production, such as the CYP enzyme inhibitor abiraterone acetate, to those that focus on metastatic bone deposits, such as the ?-particle emitter, 223RaCl2. The recent Phase III, placebo-controlled ALSYMPCA trial, which studied 223RaCl2 in men with CRPC and symptomatic bone metastases, demonstrated such a clear survival benefit that the trial was discontinued early. Nevertheless, 223RaCl2 only increased median overall survival from 11.2 to 14 months. Here, we intend to build on this initial clinical success with ?-emitter therapy by generating targeted compounds that emit ?-particles specific for both bone-localized and soft tissue-localized tumors. Alpha particles are helium nuclei that have a short but highly lethal range and have proved much more effective than ?-particle emitters in the context of molecularly targeted radiotherapy for cancer. A goal of developing ? - emitters is to minimize damage to adjacent, surrounding tissue, which is currently unattainable by standard external beam therapy. They have been employed clinically for treatment of PCa previously, but as antibody conjugates, which tend to have limited access to solid tumors. Here we will extend our program in the development of low molecular weight radiopharmaceuticals for PCa that target the prostate-specific membrane antigen (PSMA) to molecular radiotherapeutics - specifically for the synthesis and optimization of compounds for ?-emitter therapy. We have shown in clinical studies that our agents are capable of targeting both bone and soft tissue. Because PSMA is internalized upon ligand binding, fluorescent versions of our compounds localize to the perinuclear area, near the primary target for radiation therapy - cellular DNA. Preliminary dosimetry studies suggest effective treatment can be achieved with acceptable levels of radioactivity deposited to normal tissues, including kidney. We believe that treatment of metastatic CRPC is an unmet medical need for which molecular radiotherapy - particularly with targeted ?-emitters - is ideally suited. We will take a graded approach across three aims that can be summarized as: (1) synthesis of PSMA binding agents that incorporate the ?-emitter, 211At; (2) biological characterization of compounds meeting affinity and stability criteria; and, (3) radiotherapy of suitable leads in relevant mouse models of PCa and comparison of relative biological effectiveness of the top lead, and its 131I-labeled analog, to the emerging clinical PSMA-targeted radiotherapeutic, the humanized anti-PSMA antibody, 177Lu-J591. The goal is to have a lead compound at the end of this funding period ready to progress through the necessary steps for translation to patients with CRPC and biochemical-only recurrence.